Lactic acid is widely used in food, pharmaceutical, chemical industries and other technical fields. In food industry, lactic acid can be used as acidulant, emulsifier and preservative because it is a nontoxic food additive. In pharmaceutical industry, lactic acid can be used to disinfect operating rooms and other places; lactates and lactic acid esters can be used as drugs; especially polylactide is biocompatible so it can be used to make artificial limb. The artificial limb is of high strength, and the probability of inflammation and infection is low when applied to human body. In chemical industry, lactic acid is an important platform chemical, because it can be transformed into other useful chemicals, such as acetaldehyde, propanediol, propanoic acid and pyruvic acid, etc. Additionally, lactic acid can be used as skin brightener, moisturizer, antimicrobial and stabilizer. During recent years, there has been a great demand for the biodegradable polylactide. There is a large gap in lactic acid production worldwide.
The traditional method for preparing lactic acid is based on the anaerobic fermentation of corns, rice, wheat, and so on. It is the main method of lactic acid production at present. The major disadvantages associated with this method are the complicated process, the long production time and the inevitable need of adding acid alkali and other substances during the process. Meanwhile, the same amount of calcium lactate is produced with the production of lactic acid, thus it is of low efficiency and not environmental friendly. Lactic acid can also be produced through chemical composition of precursors including lactonitrile, acrylonitrile and propanoic acid. However, the method of lactonitrile needs to use hydrogen cyanide which is highly toxic, while acrylic acid and propanoic acid are expensive, so they can't be used for large-scale industrial production.
During the recent years, researchers are trying to look for suitable catalysts to produce lactic acid from biomass which is naturally abundant. But there are still two main disadvantages. Firstly, the yield of lactic acid can be improved by adding alkali, which however requires the vessel to be resistant to alkali. Additionally, the addition of alkali makes the separation of lactic acid more difficult and it does damage to the environment inevitably. Secondly, the yield of lactic acid is low, which limits its application. For these reasons, researchers have turned to the research of the derivatives of lactic acid, such as methyl lactate, achieving high yields. Denmark patent (PA 200801556, PA 200900757) first reported a one-step method to prepare lactic acid through glucose, fructopyranose and sucrose, etc. by solid Lewis acidic catalysts. The highest yield is up to 30%, which aroused worldwide attention. However, the catalysts were synthesized according to the hydrothermal method reported by the US patent (U.S. Pat. No. 6,306,364) which is complicated and costs up to 20 days. Importantly, the yield of lactic acid is rather low from the perspective of practical application Additionally, toxic hydrofluoric acid is used as mineralizer. I Comparing with traditional hydrothermal method, solid ion-exchange method has the advantages of less time consuming, simple operation, so it is suitable for large scale production.